Code

private void ReflectHorizontally(object sender, RoutedEventArgs e)
{
    var matrixTransform = new MatrixTransform();
    var trs = CreateTRSMatrix(new Vector2(0, 0), rotation: 0, new Vector2(-1, 1));
    matrixTransform.Matrix = new Matrix(trs.M11, trs.M12, trs.M21, trs.M22, trs.M31, trs.M32);

    var imageNode = diagram.Nodes.First();

    imageNode.Transform = matrixTransform;
}

private void ReflectVertically(object sender, RoutedEventArgs e)
{
    var matrixTransform = new MatrixTransform();
    var trs = CreateTRSMatrix(new Vector2(0, 0), rotation: 0, new Vector2(1, -1));
    matrixTransform.Matrix = new Matrix(trs.M11, trs.M12, trs.M21, trs.M22, trs.M31, trs.M32);

    var imageNode = diagram.Nodes.First();

    imageNode.Transform = matrixTransform;
}